1. Field of the Invention
The present invention relates to apparatus for the measurement of changes in angular orientation. More particularly, the invention relates to optical apparatus which is useful for high accuracy angular displacement metrology using interferometry.
2. The Prior Art
High accuracy linear displacement and angular displacement measurements are required in the machine tool industry and in the semiconductor fabrication industry. Linear displacement is commonly measured with an interferometer. Angular displacements are commonly measured with either an interferometer or an autocollimator.
There are numerous interferometer configurations which can be used to measure changes in angular orientation. In conventional interferometers, changes in angular orientation between two mirrors manifests itself, in general, as a change in fringe spacing and a rotation of the fringe pattern while changes in the linear displacement between the two mirrors manifests itself as a translation of the fringes. Thusly, it takes rather complex, time consuming processing to separate these effects in order to extract the desired angular information. Also, these interferometers have a very limited angular displacement measurement range. Therefore, for for angular displacement measurements which require high accuracy and a large measurement range, these prior art interferometers are not useful.
An adaptation of a linear displacement interferometer has been used to make angular displacement measurements, see for example R. R. Baldwin, L. E. Truhe, and D. C. Woodruff, "Laser Optical Components for Machine Tool and Other Calibration," Hewlett-Packard Journal, pp. 14-16, April 1983. While this technique is useful for some applications, it is impractical to use this technique of attaching several massive retroreflectors to a rapidly rotating shaft in other applications.
Although autocollimators provide many of the desired characteristics, see for example D. Malacara, Optical Shop Testing, p. 467, John Wiley & Sons (1978), for high accuracy angular displacement measurements, interferometers are preferred because their measurements are based directly on a stable, fixed, built-in measurement unit, i.e., the wavelength of light, and they have a large measurement range.
Another technique, which measures the changes in angular orientation between two plane mirrors, is disclosed in the commonly owned copending U.S. Patent applications of Gary E. Sommargren, both entitled "Angle Measuring Interferometer," and bearing U.S. Ser. No. 845,926 filed Mar. 28, 1986, and Ser. No. 873,420, filed June 12, 1986, the contents of which are specifically incorporated by reference herein in their entirety. While this technique is quite useful for many applications and overcomes several disadvantages of the prior art, it may have a limited measurement range, such as e.g. .+-.1/2 degree and, therefore may not be as universal as desired.
The present invention, however, retains the preferred characteristics of both the autocollimator and the interferometer over a large measurement range while avoiding the serious limitations of prior art apparatus. In the present invention, the angular displacement measurement has a large measurement range, high resolution, a high slew rate, and is insensitive to linear displacements of the rotating optical element in any of three dimensions. The measurement of the present invention is also interferometric so that it is based on the wavelength of light. The improvements of the present invention thusly overcome the disadvantages of the prior art and allow the high accuracy, i.e., to a small fraction of an arc second, angular displacement measurement required for precision, high speed optical scanners within a large measurement range.